1. Field of the Invention
The present invention relates to signal amplifiers and more particularly to such amplifiers which produce an output voltage signal proportional to an input current signal and which are generally known as transimpedance amplifiers.
2. Description of the Related Art
Prior art transimpedance amplifiers are typically implemented by providing a feedback resistor across the input and output of a voltage amplifier. The current signal applied to the amplifier input is thus passed substantially through the feedback resistor because of the high input impedance of the voltage amplifier. A voltage proportional to the input current thus appears on the amplifier output.
One such prior art transimpedance amplifier shown in FIG. 5 of U.S. Pat. No. 4,540,952 issued to Williams for a nonintegrating receiver.
A typical use for transimpedance amplifiers is in optical transmission systems. An optical source, such as a laser or light emitting diode, is used to transmit information by modulating the intensity of the light source. An electrooptic device such as a PIN diode or avalanche photodiode receives the optical signal and converts the same to a current which is applied to the input of the transimpedance amplifier. The amplifier thus produces a voltage proportional to the diode current. In optical transmission systems the information transmitted is usually digital and generally is in the form of a pulse train.
Optical transmission systems are typically designed to provide an electrical output signal which has a predetermined amplitude. In the prior art, an automatic gain control (AGC) is used in conjunction with the transimpedance amplifier described above in order to produce an output voltage which comprises a pulse train switched in accordance with the optical source and having a substantially constant amplitude when the input signal is larger than a preselected threshold. Increasing the gain for low signal inputs thus increases the dynamic range of the amplifier.
One such AGC circuit is described in U.S. Pat. No. 4,415,803 to Muoi for an optical receiver with improved dynamic range. Such an AGC circuit effectively varies the gain of the transimpedance amplifier in order to maintain the output signal at the desired amplitude for signals above the threshold. In transimpedance amplifiers which are implemented with voltage amplifiers, varying the value of the feedback resistance varies the amplifier bandwidth. The closed-loop -3db bandwidth for such an amplifier is approximately equal to A.sub.v /(2.pi.R.sub.f C.sub.t), where A.sub.v is the amplifier gain, R.sub.f is the value of the feedback resistance and C.sub.t is the total input capacitance of the diode, the amplifier and any other parasitic capacitance. The bandwidth of the amplifier thus varies with the value of the feedback resistance as in several embodiments disclosed in the Williams patent.
Should the bandwidth fall too low, undesirable filtering of the signal occurs. On the other hand, if the bandwidth becomes too high, overshoot, ringing, and, in extreme cases, amplifier oscillation occurs, thus producing at the very least, an increase in the error rate of information passed. Furthermore, increased bandwidth also increases noise and therefore the error rate. It would thus be desirable to provide a transimpedance amplifier having a variable gain and a constant bandwidth.
In another prior art device for increasing the dynamic range of an amplifier, a FET is used as a resistive shunt to reduce the value of the current applied to the input terminal of the amplifier in order to increase dynamic range. In such a circuit, one side of the FET is connected to the amplifier input, the other side is connected to a bias voltage and the gate is connected to the output of an AGC circuit which produces a control signal which varies in response to the amplifier output. For high input signal levels, more amplifier input current is shunted thereby increasing the dynamic range of the amplifier. In order to be effective, such a FET must typically have a resistance of 10 ohms or less. Such a FET is typically one to several thousand microns wide. This takes up a tremendous amount of space on a chip and also adds a large amount of unwanted capacitance which lowers the bandwidth and degrades the noise performance.